The present invention relates to a magnetic coupling mechanism for use in a laser apparatus. More particularly, the present invention relates to a coupling mechanism for driving a gas-circulating fan for an excimer laser.
In an electric discharge pumping laser apparatus, a fan for circulating a laser gas is disposed in a container having a laser gas sealed therein, and it is necessary to transmit rotational force from a motor disposed outside the container to the fan. For this purpose, a magnetic coupling mechanism capable of transmitting rotational force without having mechanical contact has heretofore been used.
FIG. 3 shows schematically the arrangement of a laser apparatus using a conventional magnetic coupling mechanism. Part (a) of FIG. 3 is a sectional view taken along a plane perpendicular to the direction of laser oscillation, and part (b) of FIG. 3 is a fragmentary sectional view taken along a plane parallel to the direction of laser oscillation, showing a magnetic coupling part. A container 10 is provided therein with a fan 11 for forming a stream 1 of a laser gas (in the case of an ArF excimer laser, a mixed gas of Ar gas, F2 gas and Ne gas; in the case of a fluorine laser, a mixed gas of F2 gas and He gas, for example). The container 10 is further provided therein with electrodes 2a and 2b for pumping the laser gas, heat exchangers 3, etc. The electrodes 2a and 2b are provided in the upper part of the container 10 at respective positions which face each other to perform electric discharge necessary for laser oscillation. The fan 11 is placed in the lower part of the container 10 to circulate the laser gas in the container 10. The laser gas heated by electric discharge between the electrodes 2a and 2b is agitated by the fan 11 to induce a gas stream 1. The gas stream 1 circulates in the container 10 while being cooled by the heat exchangers 3 provided in a side portion of the container 10.
A motor 15 is installed outside the container 10. An atmosphere-side magnetic joint 4 is secured to the distal end of a rotating shaft (motor shaft) 20 of the motor 15. A gas-side magnetic joint 5 is secured to the distal end of a rotating shaft (fan shaft) 12 of the fan 11. The magnetic joints 4 and 5 are members having respective permanent magnets therein. The magnetic joints 4 and 5 are disposed to face each other across a partition 6 of the container 10. Rotational force of the rotating shaft 20 of the motor 15 is transmitted to the rotating shaft 12 of the fan 11 through magnetic coupling between the permanent magnets of the magnetic joints 4 and 5. Thus, the fan 11 is driven to rotate (for example, see Japanese Utility Model Application Unexamined Publication (KOKAI) No. 6-45358).
In the above-described magnetic coupling mechanism, a metal is used as the partition 6 for dividing the atmosphere side and the laser gas side from each other. However, when a metal is used as the partition 6 between the magnetic joints 4 and 5, eddy currents are induced in the partition 6 as the magnets rotate, causing heat to be generated. Therefore, the rotational force transmission efficiency is unfavorably low, and the structure becomes undesirably complicated because of the need for a cooling mechanism and so forth. In addition, because coupling force between the magnetic joints 4 and 5 acts in the thrust direction (axial direction), bearings for supporting the rotating shafts 20 and 12 are likely to be damaged.
In the prior art, further, the magnet used in the magnetic joint 5 placed in the laser gas is plated with nickel and coated with a fluorocarbon resin material to provide corrosion resistance to F2 gas in the laser gas. However, it is difficult to plate the magnet completely. Therefore, the plating or the like flakes off with time, and hence the magnet corrodes. This causes the laser gas to be contaminated. In addition, flakes from the plating are likely to enter the space between the partition 6 and the magnetic joint 5, causing an operational abnormality.
In view of the above-described problems associated with the prior art, an object of the present invention is to provide a magnetic coupling mechanism for use in a laser apparatus, which is designed so that it can transmit rotational force of a motor with high efficiency and is simple in mechanism and unlikely to damage bearings, etc. and also unlikely to contaminate a laser gas, and flakes from plating or the like will not cause an operational abnormality.
To attain the above-described object, the present invention provides a magnetic coupling mechanism for use in a laser apparatus, which is adapted to transmit rotational force from a shaft outside a laser chamber filled with a laser gas to a shaft of a gas-circulating fan in the laser chamber to rotate the gas-circulating fan. A first magnet group is provided on an outer peripheral surface at one end of either of the two shafts. A second magnet group is radially coupled to the first magnet group through a ceramic partition constituting a part of the wall of a container of the laser chamber. The other shaft has the second magnet group provided on an inner peripheral surface thereof.
In this case, it is desirable that the surface of each magnet in the magnet group provided on the shaft in the laser chamber should be plated, and a surface of each magnet in this magnet group exposed in the laser chamber should be covered with a metal member.
It is also desirable that the thickness of the ceramic partition should be within the range of from 1.5 to 10 millimeters.
It is also desirable that the ceramic partition should be made of aluminum oxide, and the purity thereof should be not lower than 99.5%.
In the present invention, the magnetic coupling mechanism includes a first magnet group provided on an outer peripheral surface at one end of either of two shafts and a second magnet group radially coupled to the first magnet group through a ceramic partition constituting a part of the wall of the container of the laser chamber. The other shaft has the second magnet group provided on an inner peripheral surface thereof. Therefore, when rotational force is coupled, no eddy currents are induced in the partition, and no heat is generated. Accordingly, there is no reduction in rotational force transmission efficiency, and there is no need to provide a complicated mechanism, e.g. a cooling mechanism. Moreover, because force produced between the first magnet group and the second magnet group acts in the radial direction, but not in the thrust direction, there is no likelihood that bearings, etc. will be damaged by the force. If a high-purity aluminum oxide having a purity not lower than 99.5% is used for the ceramic partition hermetically dividing the laser gas side and the atmosphere side from each other, no impurities will be generated, which would otherwise contaminate the laser gas. If the surface of each magnet in the magnet group provided on the shaft in the laser chamber is plated and a surface of each magnet in this magnet group exposed in the laser chamber is covered with a metal member, the magnet group does not directly contact the laser gas. Even if the plating provided on the magnet surface becomes likely to flake off as time goes by, flakes from the plating will not enter the space between the partition and the laser chamber-side rotating member to cause an operational abnormality. Accordingly, it is possible to perform a stable operation for a long period of time.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.